1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to an optical scanning apparatus suitable for use in a multi-laser beam printer in which a plurality of modulated light beams are introduced through light deflectors onto positions on an original which are spaced in a subscanning direction so as to scan the original to form an image of the original.
2. Description of the Related Art
Optical scanning apparatus have a wide use in image information apparatuses, image processing terminal devices and so forth. Recently, various types of multi-beam scanning apparatuses have been proposed in which a plurality of scanning light beams are used simultaneously, in place of the single scanning light beam used in a conventional apparatus.
In particular, an optical scanning apparatus which employs a plurality of light beams for simultaneously scanning different portions of an original is finding increasing use as being suitable for use in multi-color printers and full-color printers.
Obviously, a multi-beam type scanning apparatus capable of scanning an object simultaneously with a plurality of beams is obtainable by combining a plurality of optical scanning devices each being capable of emitting a single scanning beam. Such a design, however, is not preferred because the size of the whole apparatus becomes impractically large. To obviate this problem, proposals have been made for a multi-beam type scanning apparatus which is capable of performing simultaneous scanning with a plurality of scanning beams using a single optical scanning unit.
For instance, the specification of U.S. Pat. No. 4,561,717 discloses a multi-beam scanning apparatus which employs a single light deflector. More particularly, this patent proposes the use of a common light deflector which is composed of a polygon mirror and an f-.theta. lens and which deflects a plurality of scanning light beams. The patent also proposes the use of a cylindrical lens disposed in the vicinity of an object and intended for correcting image inclination caused by the scanning.
With this arrangement, undesirable scanning line curvature caused by the common use of a single f-.theta. lens, is eliminated by the use of a cylindrical lens. This arrangement, however, has many disadvantages. For example, the cylindrical lens must have a large length corresponding to the number of the scanning lines and must be placed in the vicinity of the scanning object, with the result that the design of the apparatus is undesirably restricted.